Since the lead-free process of the solder, there are a variety of lead-free solders appeared on the market. However, disadvantages such as higher percentage of Sn (more than 95 wt %), higher melting point, and higher operating temperature than conventional Sn—Pb solder exists in the lead-free solder, which leads to largely increased amount of dross, impaired welding quality and enormous economic losses.
At present, various measures are adopted in the art to reduce dross with regard to different process features of wave soldering, dip soldering, and hot air leveling, which, for example, comprise (1) being coated with an oxidation resistant oil: the oxidation resistant oil is non-ion surfactant, which reacts through free radicals with ozone to reduce the oxidation of Sn at high temperature, but is not recommended in the art due to the disadvantages of deterioration after long time of use, contaminating the circuit board, big smoke in soldering furnace, and bad atmosphere for worker; (2) nitrogen protection: it has the advantages of high effectiveness and low amount of dross, as well as the disadvantages of increasing solder balls existed on the surface of the circuit board, increased requirement for the purity of nitrogen and large equipment investment; 3) improved wave soldering unit structure—electromagnetic pump: it has the advantages of no intense mechanical agitation, or strong vertex and rolling liquid surface in the furnace, which reduced the phenomenon of oxygen uptake, but has the disadvantages of instable operation, high cost, and increased equipment cost by 30-50% per unit; 4) anti-oxidation elements added to the alloy: P, Ga, Ge, and the like, which has the advantages of good effect, well established technique, easy operation, and cost effective, but has the disadvantages of short anti-oxidation life of product—the so-called not-long-effectiveness, that is, the reducing dross effect varies from goodness in the beginning to none after a short period of time, which largely impairs the practical application effect of the method.
Each of the reducing dross methods has its own scope of application, and the fourth method is considered the best among them taking all factors into consideration. More satisfied effect can be achieved if it is improved in terms of its disadvantages.